1. Field of the Invention
The present invention relates to a process for the production of L-threonine (hereinafter referred to as threonine) by a fermentation technique.
2. Description of the Background
Threonine is an important amino acid which is used in animal feeds and medicines. One process which is known for its production is a fermentation method. In this technique a mutant of the genus Brevibacterium having resistance to bacterial .alpha.-amino-.beta.-hydroxyvaleric acid (hereinafter referred to as AHV) which is a threonine analog (Agric. Biol. Chem., 34 (3) 448-456 (1970), Japanese Patent Publication No. 26708/70) is cultured. This process employs as an AHV-resistant mutant, a strain having homoserine dehydrogenase (hereinafter referred to as HD) in which feedback inhibition due to threonine has been removed (J. Biochem., 68, 859-866 (1970)). The yield of threonin produced by this process is low, and therefore, it is not an economical method for the production of threonine which is added to animal feeds. Accordingly, in order to enhance the yield of threonine, a mutant which is frequently used is one which is a lysine-producing strain. However, in this case, lysine is frequently produced as a by-product, which adversely affects the yield of threonine. Further, the separation of lysine from the medium is a complicating factor. A need therefore continues to exist for a method of producing threonine in improved yields by a fermentation technique.